1. Field of the Invention
This invention relates to the remediation of contaminated subsurface material, utilizing a composition of alkaline solids for adjusting and maintaining the pH of subsurface material to a value which enhances remediation.
2. Description of the Related Art
There are numerous techniques employed for the remediation of contaminated subsurface material. The mechanisms for cleanup may be physical, chemical or biological. Common physical remediation methods include excavation and disposal of contaminated soil, and pumping and treatment of contaminated groundwater.
In situ treatment of contaminated subsurface material is often a less expensive approach because it eliminates the need for physical removal of the contaminated material. Common in situ treatment approaches include aerobic and anaerobic bioremediation, chemical oxidation and reduction, soil vapor extraction, air sparging, and in situ stabilization-immobilization. Most, if not all, in situ treatment processes have an optimum pH for the treatment process. Many bioremediation processes require a pH of between 6 and 8 Standard Units (SU) for optimum growth of the required microorganisms and contaminant biodegradation. Chemical oxidation, reduction and immobilization processes will also have an optimum pH. If the pH is too low, reaction rates may be reduced or the solubility of the target chemical may be too high or too low. Different remediation techniques that have been employed for various contaminants are discussed more specifically below.
It is noted that in discussing related art herein, it is often referred to in somewhat cryptic notation. For purposes of clarity, reference is made to a bibliographic section set forth at the end of this BACKGROUND OF THE INVENTION where full citations of references discussed and other references of interest are identified in their entirety. The inclusion of a reference in the bibliographic section and/or in any discussion of related art is not intended to suggest that all such references do or could constitute prior art with respect to the present invention, as certain references are included and/or discussed simply to provide a broader appreciation of the art.
The optimum pH for microbial growth is dependent on the specific microorganisms and their respiration pathways. Aerobic microorganisms often tolerate a wider range in pH, whereas many anaerobes are sensitive to pH and operate efficiently only in a narrow pH range. Denitrification and methanogenic biodegradation rates are usually optimum between a pH of 7 and 8 SU, and may drop off rapidly below a pH of 6 SU (van den Berg, 1974; US EPA, 1975). The pH of most water supply aquifers is between 6.0 and 8.5 SU, although water having lower pH is not uncommon (Hem, 1999).
While microbial populations can endure a wide range of pH, a pH close to neutral (6 to 8 SU) is the most conducive to the growth and proliferation of healthy and diverse microbial populations necessary for anaerobic dechlorination. Low pH conditions (<5 SU) are detrimental to sulfate-reducing, methanogenic, and dechlorinating bacteria. Dehalococcoides ethenogenes are the only known organisms that can completely dechlorinate perchloroethene (PCE) and trichloroethene to the non-toxic endproduct ethene. However, Dehalococcoides E. appear to be very pH sensitive. Young and Gossett (1997) found that dechlorination of PCE was four-fold slower at pH 6 than at pH 7 SU in a series of experiments with an enrichment culture known to contain Dehalococcoides. 
A variety of heavy metals can be immobilized in situ by increasing the aquifer pH. Barium (Ba), cadmium (Cd), chromium (Cr), lead (Pb), and mercury (Hg) have a reduced solubility under alkaline conditions (Dragun, 1988) so these metals can be precipitated in situ by adjusting the pH. Other contaminants including arsenic can be treated by enhancing iron (Fe) or manganese (Mn) precipitation through pH adjustment. In addition, heavy metal removal can be enhanced by adjusting the pH to enhance sorption to mineral surfaces including iron, manganese, alumina, silica oxides and their respective hydrous, anhydrous hydroxy, and oxyhydroxy forms (Bethke, U.S. Pat. No. 7,141,173, November 2006).
Heavy metals can be further reduced using a combination of pH and redox adjustment. Deutsch et al. (2002) describe the enhanced removal of Fe and As induced by addition of an oxidizing agent and alkaline material. Miller et al. (2006) demonstrated that addition of dissolved NaOH could be used to increase the pH of acidic groundwater (pH 3 to 4 SU), reducing levels of dissolved cadmium, copper (Cu), lead, manganese, nickel (Ni), and zinc (Zn). However, use of calcium polysulfide (CPS) in combination with sodium hydroxide (NaOH) was most effective in treating severe conditions.
Chemical oxidation processes can be used to treat subsurface material and groundwater contaminated with organic and inorganic pollutants. Many of these processes have an optimum pH for destruction or immobilization of the pollutants. For example, chemical oxidation in combination with pH adjustment can be used to precipitate iron, manganese and arsenic (Hem, 1999). Persulfate in combination with high pH can be used to chemically oxidize a variety of subsurface contaminants including chlorinated ethenes, ethanes, and methanes, mono- and polynuclear aromatic hydrocarbons, oxygenates, petroleum hydrocarbons, chlorobenzenes, phenols, pesticides, herbicides, ketones and polychlorinated biphenyls (FMC Environmental Solutions, Klozur Activation Chemistries, 2006; Block et al., 2006, US Patent Application 20060054570, ITRC, 2006; Brown et al., 2006; White et al. 2006; Crimi and Taylor, 2006). However, pH levels greater than 10.5 SU are required activate persulfate enhancing oxidative degradation of many target compounds (ITRC 2006; Crimi and Taylor, 2007). Achieving these high pH levels can be difficult due to the strong buffering capacity of many subsurface materials. Block et al. (2005) describe a process for oxidizing organic compounds where the organic compound is contacted with a composition of a water soluble peroxygen and a water soluble pH modifier (e.g. sodium and potassium hydroxide), which maintains the pH of the composition at greater than about 10 SU). However, a solid alkaline material such as CaO or Ca(OH)2 could also be used to increase the pH to greater than 10 SU.
Chemical reduction processes can also be used to treat subsurface material and groundwater contaminated with organic and inorganic pollutants. For example, Boparai et al. (2006) showed that aquifer sediment and surface soils contaminated with herbicides can be treated with dithionite when the pH is increased to 8.5 SU. However, at the ambient pH of 6.9 SU, there was no transformation of the pollutant. Similar results were reported by Lee and Batchelor (2002) who reported an increase in the TCE dechlorination rate when the pH was increased from 6.8 to 8.1. A pH of 8.1 to 8.5 SU could be achieved by injection of a suspension of Mg(OH)2.
There are a variety of different conditions that can lead to low pH conditions which can inhibit treatment processes. In the Southeastern United States, many soils and the underlying aquifers have a naturally low pH. Under anaerobic conditions, a variety of organic materials can be fermented, releasing short-chain fatty acids (butyric, propionic and acetic acids) that can further reduce pH. Frizzell et al. (2004) found that injecting a mixture of high-fructose corn syrup and cheese whey stimulated biological activity resulting in a drop in pH to below 4.0 SU.
Currently, there are two generally accepted available methods for increasing aquifer pH. The first and most common method is to circulate a solution containing a dissolved base or alkaline material through the treatment zone. Materials commonly used include aqueous solutions of NaOH, potassium hydroxide (KOH), sodium carbonate (Na2CO3), sodium bicarbonate (NaHCO3), and sodium metasilicate (Na2SiO3). Arcadis (2002) and Lutes et al. (2006) describe methods for circulating buffer solutions containing carbonates, bicarbonates, or phosphates to control pH declines. Cline et al. (2005) describes injection of KOH solutions to increase the aquifer pH from 4.5 to as high as 6.6 SU to enhance reductive dechlorination of PCE at a dry cleaning store.
While circulating alkaline solutions through the treatment zone can be effective, there are some major disadvantages to this approach. As the alkaline solution migrates through the formation, the alkalinity present in the water reacts with the acidic mineral surfaces and is consumed. Consequently, a large amount of alkaline material must be added to increase the pH. This can be accomplished by injecting multiple pore volumes of dilute base or smaller amounts of very concentrated base. Injecting multiple pore volumes is difficult to implement and increases costs. Injecting very concentrated base will increase the pH to unacceptable levels and can expose site workers to safety hazards.
A second approach for increasing the pH of the formation is to inject a solid alkaline material. These materials can be injected by boring a hole in the subsurface followed by gravity or pressure injection of a slurry. Solid alkaline materials that can be used in this approach include magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCO3), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), and calcium carbonate (CaCO3). Deutsch et al. (2002) describe the injection of a slurry of MgO and Mg(OH)2 to increase the pH and redox potential to precipitate iron and arsenic. While this approach has the advantage over aqueous injection in that large amounts of material can be quickly injected, the beneficial increase in pH is often limited to the immediate area around the injection point. To increase the pH throughout the treatment zone, the solid alkaline material (also referred to as “alkaline solids”) can be physically mixed with the sediment or transported through the pore spaces away from the injection point. Large augers or mixers can be used for physical mixing, which typically is very expensive and disruptive.
Stanforth et al. (U.S. Pat. No. 5,202,033) teach a method of immobilizing heavy metals in soil or solid waste by mixing with a phosphate, carbonate, or ferrous sulfate stabilizing agent and alkaline material including MgO, Mg(OH)2, CaO and Ca(OH)2. The stabilizing agent and alkaline material may be physically mixed with the soil at the surface or injected as a solution or slurry using injection wells or an injection nozzle. Stanforth teaches a variety of methods for physically mixing the material with the sediment including tilling and in-place mechanical mixing with a hollow-stem auger. Stanforth does not make any mention of methods to transport solid alkaline materials through the aquifer pore spaces or that this transport mechanism would be possible.
There are several major challenges associated with transporting alkaline solids through the aquifer pore spaces including particle removal by settling, capture of positively charged alkaline solid particles by negatively charged soil surfaces, and particle-particle collisions which lead to formation of dendrites that reduce transport and eventually lead to pore clogging.
Transport of solid alkaline particles is reduced by settling of the particles in the aquifer pore spaces. Removal of solid particles by settling is proportional to the particle specific gravity, aqueous phase viscosity, settling distance, and particle diameter (Fair et al., 1968). All useful alkaline solids of which the inventor presently is aware have a specific gravity greater than 2 which can result in rapid settling. In the subsurface, particle removal efficiency will be very high in due to the very small distance a particle must settle in an aquifer pore. Particle capture by settling can be reduced by adding thickening agents to the injection fluid to increase viscosity. However, the increase in fluid viscosity makes injection more difficult (Cantrell et al., 1997). Particle removal by settling can be reduced by using very small particles. However as particle diameter decreases, Brownian motion increases particle-aquifer collisions increasing removal (Tratnyek and Johnson 2006). As a result, there will be an optimum particle diameter where both settling and particle collisions due to Brownian motion are minimized.
Borden and Lee (U.S. Pat. No. 6,398,960B1) teach a method for remediating contaminated aquifers with an oil emulsion with an average droplet size less than the median pore size of the sediment to reduce straining capture in the aquifer pores. Coulibaly and Borden (2004) present pore size distributions for a variety of sediments where the median pore size varies from 35 to over 100 microns, indicating the average droplet size of the emulsion would need to be less than 30 microns. Borden et al. (U.S. Pat. No. 6,398,960) do not make any mention of settling as an important control on emulsion transport in the subsurface since oil droplets are slightly less dense than water and settling has no significant impact on emulsion transport.
The point of zero charge for useful alkaline solids (including MgO, Mg(OH)2, MgCO3, CaO, Ca(OH)2, and CaCO3) varies between 8 and 12 (Pechenyuk 1999, Parks 1965, Pokrovsky et al. 1999), so these solids will have a positive surface charge under ambient conditions. Under these same conditions, most aquifer surfaces have a net negative charge. As a result, alkaline solids are strongly retained. Seaman et al. (U.S. Pat. No. 5,846,434) teach a method of mobilizing colloidal metal oxides from a contaminated aquifer by flushing a solution containing a cationic surfactant, preferably a quaternary alkylammonium surfactant, through the aquifer. The cationic surfactants adsorb to the surface of negatively charged phyllosilicate clays, generating a positively charged surface, reducing capture of the metal hydroxides. Seaman et al does not make any mention of altering the surface charge of the mobile particles or controlling the settling rate in the subsurface to improve particle mobility. However, Seaman et al. does describe the addition of alkaline material to extracted water to cause the colloidal metal oxides to settle out of the extracted groundwater. Seaman's addition of alkaline material to enhance settling illustrates the challenges of distributing solid alkaline material in the subsurface since these materials will rapidly settle out in the aquifer pore spaces and will not be transported any significant distance.
Particle-particle collisions often lead to formation of dendrites that reduce transport and result in pore clogging (Soo and Radke 1985, Rege and Fogler 1988, and Sahimi and Imdakm 1991). A variety of different investigators have attempted to improve transport of nano and micron sized particles using a variety of different coatings and stabilizing agents to reduce particle-particle collisions and dendrite formation (He et al. 2005, Hydutsky et al. 2007, Quinn et al. 2005, Saleh et al. 2007, Schrick et al. 2004). However none of these methods have been satisfactory. Gavaskar, et al. summarize the results of a series of field tests where effective distribution of nano-particles was not successful.
With reference to the foregoing discussion, reference is made to the following references, the disclosures of which are specifically incorporated by reference herein.
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